The adaptive or acquired immune system is the body’s main line of defense against ever-evolving pathogens. This subsystem of over all immune system comprises of specialized cells to eliminate lethal organisms which otherwise may lead to death. Read on to find where the cells of the adaptive immune system reside, and what their mechanisms of action are.

00:01
Because lymphocytes recombine their antigen receptor
genes in a random way, it means that we can producemillions and millions and millions of different
B-cell receptors on the surface of B-cells;and millions and millions of different
T-cell receptors on the surface of the T-cells.
00:19
So that’s great, means that whatever
pathogen comes along, if somenew pathogen arises or if an existing pathogen undergoes mutationand changes its antigens, we should have some lymphocytes thathave an antigen receptor specific for that particular pathogen.
00:36
However, that comes at a price.
00:38
This huge diversity of T-cells and huge
diversity of B-cells means that for anygiven single pathogen, we don’t have
millions and millions and millions of cells.
00:48
We may only have a few
hundred or a few thousand.
00:51
It’s actually not enough to do the
job of getting rid of the infection.
00:55
So there’s a problem there.
00:57
So how does the immune system
get around that problem?Well, it does it by clonally selecting
the B-cells and the T-cells that arerequired, then activating them and causing
them to divide and divide and divide.
01:12
So that particular
specificity will expand up.
01:15
So rather than having just a few thousand, let’s say T-cellsand B-cells against Streptococcus for example; rather than justhaving a few thousand, you
end up with millions, and that isenough to do the job that’s needed to eliminate the infection.
01:28
So clonal selection of lymphocytes is key to the way
in which the adaptive immune response functions.
01:35
So here we have a B-cell in the middle, say five differentB-cells with five different specificities of B-cell receptor.
01:42
And this one is specific for that
particular virus that we can see there.
01:48
And it will become activated by the mechanisms that we’ve justreviewed, and part of that activation
will initiate cell proliferation.
01:58
The cell will divide and it will divide and
it will carry on dividing multiple times.
02:04
And expand up the number of cells so that a sufficient
number is generated to eliminate the particular infection.
02:17
Once the cells become activated, not only do
they proliferate, but they also differentiate.
02:23
So a naïve B-cell, one that has not
previously encountered the antigen,if it encounters the antigen it’s
specific for, it will become activated.
02:34
As part of that activation, proliferation
will be induced following clonal selection.
02:40
And then the B-cell can differentiate
down one of two different pathways.
02:48
Most of the B-cells will
develop into plasma cells.
02:53
These are the antibody secreting
version of the B-cell.
02:58
So a B-cell will differentiate
into a plasma cell.
03:01
It has a rather different
appearance to the B-cell.
03:04
It has more cytoplasm, which is
why it’s called a plasma cell.
03:08
It has lots of rough endoplasmic reticulum which
is the place where proteins are made in cells.
03:13
And this particular cell is going to make lots and lots andlots and lots of one particular protein, the antibody molecule.
03:19
So these plasma cells can now secrete vast amounts of antibodyagainst the antigen that initially
stimulated that particular B-cell.
03:28
So if this B-cell was against Cytomegalovirus, you’ll
produce lots of antibody against Cytomegalovirus.
03:35
However not all of the B-cells
differentiate into plasma cells.
03:39
Some of them will differentiate
into memory B-cells.
03:43
And these cells are crucially important
in developing a secondary immune response.
03:51
Characteristic feature of the
adaptive immune response is that upona subsequent encounter with the same
antigen, the response is much faster.
04:00
And actually, not only is it faster but the
quality of the response is much better as well.
04:06
And this property depends upon
the generation of memory B-cells.
04:14
So memory B-cells provide the basis
of the secondary immune response.
04:19
Looking here on this graph, we can see the magnitude of theresponse following the first encounter with a given antigen.
04:27
The adaptive immune response develops
this primary immune response.
04:32
This term primary and secondary immune response there,
it’s uniquely used for the lymphocyte responses.
04:37
It’s characteristic of
the adaptive response.
04:40
We don’t see this in
the innate response.
04:42
The innate response is the same however
many times the antigen is encountered.
04:46
But here we see a
adaptive immune response.
04:49
And initially the primary
immune response, it’s okay.
04:53
But it doesn’t make masses of antibody that… not masses of
T-cells are produced, but it usuallyis sufficient to do the job.
But on a second encounter with the sameantigen, because during the first response
memory cells were generated, the secondresponse gets off the ground much faster.
It goes to a much higher leveland it stays up for much longer.
So both qualitatively and quantitatively, secondary immuneresponses are superior to primary immune responses.

About the Lecture

The lecture B-Cell Differentiation – Lymphocyte Activation by Peter Delves, PhD is from the course Adaptive Immune System. It contains the following chapters:

Clonal Selection of B-Cells

B-Cell Differentiation

Memory Cells

Included Quiz Questions

In comparison to the primary immune response, the secondary immune response...?

...is of greater magnitude.

...is slower to be initiated.

...is less specific.

...involves B-cells but not T-cells.

...cannot generate memory cells.

Clonal selection allows the immune system to...?

...recognize a wide variety of antigens and produce a tailored response to that antigen.

...ensure that it does not recognize self antigens.

...respond to pathogens it has encountered before.

...constantly produce antibodies to any antigen humans can encounter.

...immediately respond to antigens that it encounters.

Which of the following are the two products of B cell differentiation and their CORRECT function?

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